Enhanced High-Temperature Thermoelectric Performance of Yb_14–xCa_xMnSb₁₁

Abstract

The high temperature p-type thermoelectric material Yb(14)MnSb(11) has been of increasing research interest since its high temperature thermoelectric properties were first measured in 2006. Subsequent substitutions of Zn, Al, and La into the structure have shown that this material can be further optimized by altering the carrier concentration or by reduction of spin-disorder scattering. Here the properties of the Yb(14-x)Ca(x)MnSb(11) solid solution series where isovalent Ca(2+) is substituted for Yb(2+) will be presented. Crystals of the Yb(14-x)Ca(x)MnSb(11) solid solution series were made by Sn-flux (x = 2, 4, 6, 8) with the following ratio of elements: (14-x)Yb: xCa: 6 Mn: 11Sb: 86Sn, and their structures determined by single crystal X-ray diffraction. The density of the material significantly decreases by over 2 g/cm(3) as more Ca is added (from x = 1 to 8), because of the lighter mass of Ca. The resulting lower density is beneficial from a device manufacturing perspective where there is often a trade-off with the specific power per kilogram. The compounds crystallize in the Ca(14)AlSb(11) structure type. The Ca substitution contributes to systematic lengthening the Mn-Sb bond while shortening the Sb-Sb bond in the 3 atom linear unit with increasing amounts of Ca. Temperature dependent thermoelectric properties, Seebeck, electrical resistivity, and thermal conductivity were measured from room temperature to 1273 K. Substitution of Yb with Ca improves the Seebeck coefficient while decreasing the thermal conductivity, along with decreasing the carrier concentration in this p-type material resulting in an enhanced thermoelectric figure of merit, zT, compared to Yb(14)MnSb(11).

Cationic Site-Preference in the Yb14-xCaxAlSb11 (4.81 ≤ x ≤ 10.57) Series: Theoretical and Experimental Studies

“…The geometrical features of these four octahedra closely resemble the structural characteristics of those in the previously-reported Yb 14- x Ca x MnSb 11 series [14], as illustrated in Figure 2. For instance, the M 1-site provides the significantly distorted octahedral coordination environment, which can accommodate the relatively larger-sized element, whereas the M 4-site serves as the most symmetric coordination environment among the four cationic sites, which is suitable only for the relatively smaller-sized atom.…”

“…31%–78%, as listed in Table S1. In addition, the bond distances between Al and Sb1 in the (AlSb 4 ) 9− tetrahedron and between Sb3 and Sb4 in the (Sb 3 ) 7− are linear trimer ranged, respectively, from 2.712–2.714 Å and from 3.190–3.193 Å (Table S2), which are comparable to 2.718 Å and 3.196 Å found in Ca 14 AlSb 11 [20] and 2.749–2.762 Å and 3.169–3.183 Å in the Yb 14- x Ca x MnSb 11 (2 ≤ x ≤ 8) series [14], respectively. Moreover, despite the nearly identical ionic size of Yb 2+ and Ca 2+ cations (1.13 Å vs. 1.06 Å) [21], the increase of Ca content in the formula from 4.81–10.57 resulted in the gradual increase of lattice parameters a and c from 16.5704 to 16.6488 Å and from 22.1976 to 22.3684 Å, respectively (Table 1).…”

“…Among several Zintl phase compounds considered as potential thermoelectric materials, Yb 14 MnSb 11 is one of the best p -type TE material for the high temperature applications [10]. In order to improve its figure of merit zT value even further, various cationic or anionic element substitutions have been attempted, respectively, to lower the lattice thermal conductivity or to optimize the carrier concentration [11,12,13,14,15]. …”

Yb14MgSb11 and Ca14MgSb11—New Mg-Containing Zintl Compounds and Their Structures, Bonding, and Thermoelectric Properties

“…The longer bond lengths for the Ca compound have been explained by higher electron density on Sb transferred from Ca, although both Yb and Ca are divalent according to electron calculation. 5,10,11,25 The Yb−Sb bond lengths in Yb 14 MgSb 11 range from 3.137(2) Å for Yb(3)− Sb(3) bond to 3.432(2) Å for Yb(4)−Sb(2) bond. Two long Yb−Sb (Yb(2)−Sb (2) and Yb(4)−Sb (3)) distances are also listed in Table 2, but they are not considered to be bonding interactions.…”

“…Yb and Ca solid solution has been studied in the case of Mn-containing compounds showing an enhancement of zT. 25 Therefore, Yb and Ca solid solution in the Mg-containing compounds should also be possible without harming thermoelectric properties, thus decreasing the cost and weight of potential thermoelectric devices.…”